The present invention relates to antenna designs and, more specifically to antenna designs for data transmission which improve signal fidelity in multi-path environments.
Only recently have modern modulation techniques, made possible by recent chipsets, enabled low power short range application of radio frequency devices to become practical or economically feasible for the transfer of high speed data in the radio frequency spectra. The relatively low power levels and high frequencies used in high speed data transfers necessitate the location of antenna close to the intended targets. Most consumer devices of this type are intended to be used at less than one hundred meters. Regulatory bodies in turn place restrictions on the isotropic radiation emitted from these devices. This leads to antenna placement in confined spaces in building interiors such as closets, plenum spaces, etc.
Such enclosed spaces are characterized by much more complex boundary conditions than those for which traditional antennae or antenna arrays were designed. That is, traditional antenna designs addressed the need to transmit and receive telemetry and data effectively over relatively long distances. These legacy designs work best when positioned in an environment of high visibility (e.g., on a hilltop or radio tower) in which the antenna is exposed to low levels of multi-path signals and near field disturbance. Unfortunately, conventional antennae are now being deployed under conditions which are radically different than those for which they were designed. In addition, many antennae are housed in materials which are unsuitable for use in building or other enclosed environments in which it is necessary to keep flame spread and noxious, combustion-induced fumes to a minimum.
Legacy antenna designs deployed in their intended environments are often actually aided by the conditions under which they are deployed. For example, the undesirable multi-path signal element due to signal reflection is attenuated by distance. In addition, over long distances the angle of incidence of reflected signals will typically result in much of the unwanted reflections missing the receiving antenna. However, these conditions do not prevail in today's low power, digital environments. Moreover, because conventional antenna designs have little need to compensate for near field problems they are ill equipped to handle the near field disturbances common in such environments.
Legacy antenna designs are also typically characterized by a broad received spectra. Unfortunately, in low power, digital systems, this characteristic results in eddy currents and hysteresis losses within the transmission cable, as well as reduced sensitivity of the receiving unit.
As mentioned above, modern considerations for digital low power RF systems typically result in less than optimal antenna placement. The locations selected are strongly influenced by the structure in which the system is deployed. The structural characteristics of the deployment environment, in combination with the reactive elements of a conventional antenna, alter the effective impedance of the antenna. This in turn results in decreased performance as well as potential damage to the attached transponder.
A variety of approaches have been used to address issues relating to the use of legacy antenna designs in environments having complex boundary conditions and high multi-path. One set of solutions simply attempts to place the antennae in a high visibility locations. However, although an effective approach, many industries (e.g., hospitality, restaurant, transportation, etc.) strongly object to having antennae in view for reasons of aesthetics. Camouflaged antennae may be placed in more effective locations. However, most indoor environments provide few good options for effective placement with traditional antenna designs.
Under traditional conditions or antenna placements, an increase in antenna gain may be used to narrow the beam width of the antenna, thereby reducing the multi-path component to which the antenna is exposed. This generally requires an increase in the size of the antenna. Unfortunately, in a high multi-path environment such an increase in size is counterproductive in that a larger antenna is exposed to more of the multi-path signals in the environment.
One set of solutions for dealing with multi-path involves the use of specialty polarization schemes. One such solution involves the use of antenna diversity, e.g., port, spatial, or a combination. However, though this approach is useful in managing multi-path, it does nothing for near field problems and, in fact, presents more challenges relating to near field disturbances than the use of a single antenna. Such antenna diversity schemes also may result in a greater chance of equipment damage due to impedance mismatch. In addition, depending on the “flavor of diversity” used, the antennae have to be separated by some distance. This is often impractical in an environment which offers little space. Antenna diversity is also a relatively expensive solution in that it requires at least two antennae and their related hardware.
Circular polarization is a commonly used scheme because of its alleged “inherent immunity” to multi-path. The actually demonstrable benefit of circular polarization is that it accepts linear polarization (i.e. horizontal or vertical and their variances) more or less equally allowing for a best case majority rule. However, the tradeoffs of circular polarization include a relatively large impedance matching network making the antenna susceptible to near field problems, and a wide bandwidth making the antenna susceptible to out of band interferences.
An increase in transmit power (independent of receiver sensitivity) is often used as a means to overwhelm multi-path elements common to crowded or confined environments. Although this method allows for a smaller antenna (thus resulting in lower antenna exposure to the multi-path environment), the increase in applied power tends to exaggerate near field problems.
A variety of active signal processing techniques have also been developed to resolve source signals which are separated in phase by close and near obstructions (i.e., multi-path signals). One example, commonly referred to as MiMo (multiple-in, multiple-out), is a process which uses active components to align out-of-phase signals from a single source as experienced across multiple receiving antennae. Under MiMo, multiple traditional antennae are used and the delay spread is accounted for with complex signal processing techniques which rely on active technology. However, while such an approach may be effective in reducing multi-path for some applications, it is not economically feasible for many of the most common low power, digital systems being deployed today. In addition, MiMo designs are not particularly effective in addressing near field problems. Finally, the processing overhead required for such techniques undesirably affects data throughput.
In view of the foregoing, there is a need for improved antenna designs for use in low power, digital applications and environments characterized by high multi-path and near field problems.